Vegetative Morphology and Anatomy of the Salt Marsh Rush, Juncus Roemerianus Lionel N

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Gulf Research Reports

Volume 5 | Issue 2

January 1976 Vegetative Morphology and Anatomy of the Salt Marsh Rush, Juncus roemerianus Lionel N. Eleuterius Gulf Coast Research Laboratory

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Recommended Citation Eleuterius, L. N. 1976. Vegetative Morphology and Anatomy of the Salt Marsh Rush, Juncus roemerianus. Gulf Research Reports 5 (2): 1-10. Retrieved from http://aquila.usm.edu/gcr/vol5/iss2/1

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VEGETATIVE MORPHOLOGY AND ANATOMY OF THE SALT MARSH RUSH, JUNCUS ROEMERIANUS

LIONEL N. ELEUTERIUS Gulf Coast Research Laboratory, Ocean Springs, Mississippi 39564

ABSTRACT The extensive rhizome development found in Juncus roemerianu$ makes this species unique among rushes and is a biological feature responsible, in part, for its domination of large tracts of salt marsh. Branching in certain mature plants is distinctly sympodial, while in most it is obscured by precocious development of the continuation bud and appears to be monopodial. Each vegetative unit is composed of a scaly rhizome which grows to varying lengths and then abruptly turns up at the end to become an erect shoot. A continuation rhizome consistently arises from an axillary bud in a ventral scale leaf. Transitional leaves (large scale leaves) accompany development of the erect shoot. Rhizome scales, transitional and foliage leaves are distichously arranged and in the same vertical plane. The culm forms through an elongation of an internode of an erect shoot. Other rhizomes may also arise from buds in the axils of the transitional and foliage leaves. From one to seven terete leaves with a bifacial sheath are produced from the apical meristem of the erect stem. Fibrous roots occur laterally on erect shoots. Non-fibrous roots occur on the ventral surface to the rhizomes. The internal rhizome and root anatomy resembles that reported for most other species of Juncus while the leaf anatomy is very similar to that of Juncus maritimus and Juncus acutus.

INTRODUCTION were dug out for morphological study. Plant material was taken from more than 20 different habitats. These habitats Juncus roemerianus Scheele is a rush dominating major varied in several ways: leaves of J. roemerianus in some tracts of salt marsh on the Atlantic and Gulf coasts of the places were abundant, sparse, tall, short, in pure stands and United States (Eleuterius, manuscript in preparation). The associated with other vascular plants. Plants were also ex- most successful herbaceous plants occurring in these mari- amined from low, intermediate and hypersaline marshes, as time marshes are those which are perennial and reproduce were plants colonizing new terrain, where they grew without vegetatively by vigorous rhizome growth, thus leading to the competition. Plant material for anatomical work was fmed formation of dense stands. Once established, these species in a formalin, proprionic acid and ethyl alcohol solution do not generally rely on the sexual cycle for maintenance (FPA), processed using a tertiary butyl alcohol series and of the closed stand. The importance of understanding the embedded in the usual manner. Sections were cut 10-15 structure of herbaceous species has become apparent in the microns on a rotary microtome, affiied to slides using study of crop plants where solution of practical problems Haupt's adhesive and stained with safranin and fast green arises (Hayward 1938; Bonnett 1935, 1940). Only through using standard procedures outlined by Johansen (1940) and fundamental knowledge of the pattern of growth, entailing Sass (1958). basic morphology and anatomy, can proper analysis of variation in species be attained. Structural studies of salt VEGETATIVE MORPHOLOGY marsh species have similar importance since successful man- agement of estuaries will ultimately require familiarity with General each wild species to the same extent and detail now available for cultivated crop plants. A single mature plant of J. roemerianus is difficult or Although there are several reports (Adamson 1925; Cutler impossible to discern by cursory examination in a typical 1969; Stace 1970) on the internal anatomy of various Juncus stand. Rhizomes may connect hundreds of erect shoots of a species, comparable studies on the vegetative morphology single plant in a relatively small surface area of marsh. The are lacking. Some general morphological aspects of certain continuous proliferation and subsequent senescence of rushes are described by Buchenau (1906) and Richards and rhizomes account for the vegetative spread and maintenance Clapham (1941). Anderson (1974) in a general anatomical of the mature stand (Eleuterius 1975). Long, interconnected survey of seven salt marsh plants in North Carolina also rhizomes bearing erect shoots and roots represent a portion included descriptions of some anatomical features of J. of a single clone and serve to illustrate the typical mature roemerianus. The present study describes in detail the vegeta- plant. The vegetative axes of the mature plant are dimorphic tive morphology, pattern of growth, and anatomy of mature with a clear distinction between horizontal and erect shoots plants of J. roemerianus as found in Mississippi. (Figure 1A), but the sequence of events typical of sympodial branching is often obscured by the vigorous growth asso- METHODS AND MATERIALS ciated with the horizontal axis.

Plant portions consisting of rhizomes (subterranean seg- The Rhizome ments 30 to 60 cm in length) bearing erect shoots and roots Rhizomes possess tightly overlapping leaf-like scales or 2 ELEUTERIUS cataphylls which arise ventrally and dorsally. The plane of being about 2.5 centimeters (Figure 2C--E). distichy is vertical. The rhizome apex does not grow in- definitely but eventually becomes an indeterminate erect, The Erect Shoot leafy shoot. Young rhizome portions are generally larger in diameter than older parts, which become somewhat con- The mature erect shoot may vary from one to several stricted with age. Rhizomes range in diameter from 2 to centimeters in length and consists of a series of nodes and 18 mm, the average ones being about 9 mm. The internode compressed internodes. The growth of the erect shoot is length may vary from a millimeter or less to over a centi- determinate, culminating in the production of terete leaves meter. The rhizome bud is formed periodically in the axil of or a culm. Apparently as the erect shoot develops, the scale a scale leaf which arises on the lower surface of the rhizome leaves become longer and tougher in texture as each succes- (Figure 1B-E). The division of the rhizome apex is almost sive node is produced. These may be conveniently referred dichotomous with the resulting dorsal meristem being only to as transitional leaves, contrasting with the scale leaves of slightly larger than the ventral. The dorsal and ventral the rhizome and terete foliage leaves. There are generally meristems are destined to form the erect shoot and new three to five present on each erect shoot. The arrangement rhizome, respectively. In most instances the terminal rhi- of transitional and foliage leaves is distichous. zome bud develops precociously and continues to grow at A culm is apparently not produced on every erect shoot, such a rapid rate that the sympodial branching pattern is some of which then remain leafy throughout their existence. obscured to the extent that it may be erroneously inter- The culm is an elongated internode of the erect shoot, which preted as monopodial. The precocious growth of the rhi- elevates meristematic tissue destined to be the inflorescence. zome bud displaces the terminal bud of the previous unit, A red band circumscribes the floral-meristem (node) of the which becomes an erect, leafy shoot. This displacement culm and separates the internode region from the bract occurs in such a way that the leafy shoot appears to be which subtends the inflorescence. Culm production termi- inserted at the middle or, more often, toward the distal end nates the life of the erect shoot. However, the rhizome units of the rhizome internode. Occasionally, a rhizome may arise supporting the erect shoot may remain alive. The culm from a bud formed on the dorsal side of the rhizome apex appears externally much like a leaf; however, it is weakly which grows in a direction opposite the terminal rhizome developed and is broken more easily than the leaf. when eviction of the leafy shoot occurs. The apex of the dorsal rhizome becomes oriented in the opposite direction The Foliage Leaf from that arising from the ventral bud. As many as three rhizomes may arise from a single erect shoot. These new All foliage leaves are cylindrical or unifacial above the rhizomes may occur in the same or opposite directions. sheath with the abaxial surface to the exterior. The lower Most new rhizomes arise in leaf axils on the side of the portion or sheath of the foliage leaves is bifacial with free erect shoot synonymous to the ventral surface of the rhi- margins through which new leaves emerge. All sheaths are zome (Figure 1F-G). Deflection of apexes by obstructions rolled and the leaf margins form a slit through which sub- in the substratum causes many planes of growth or centri- sequent leaves protrude as they are produced by the apical fugal spread of the plant. Consequently, all new rhizome meristem of the erect shoot. The position of the slit alter- production is in one plane. Axillary buds of the erect shoot, nates with each successive leaf produced. Leaf production synonymous to the dorsal surface of the rhizome, give rise is two-ranked or distichous. Thus, the second leaf generally to new rhizomes which reinvade terrain previously occupied emerges from a slit on the abaxial surface of the first leaf. by the plant (Figure 2A and B). As many as seven leaves may arise sequentially from the The number of nodes or internodes between erect shoots relatively short erect shoot. The upper portion of terete is variable. In some localities the number may vary from 10 leaves are stiff and pointed at the end, and may range in to 18, in others 6 to 12, or 1 to 5. The number varies within the same clone. However, most material examined ranged diameter from a few millimeters to three-quarters of a from 8 to 14 and averaged about 12 nodes between erect centimeter; the average is about 3 millimeters. Mature shoots. In seedlings only one node may exist between erect leaves range in length from 30 to 223 cm, depending on shoots (Eleuterius 1975). The distance between erect shoots the habitat. Over most of the local population they average may be minute to 5.5 centimeters or more, the average about 100 cm.

Figure 1. Vegetative morphology of Juncus roemeriunus. 1A. A general view of a portion of a mature plant. All units of the sympodium shown are connected and may be traced back to the parental unit on the right. Note that the orientation of most new vegetative units occurs in the same direction with respect to the parental unit (i = inflorescence). 1B. Rhizome apex (a) after forming in this case after five nodes and internodes were produced. IC. Erect shoot (es) forming by eviction and precocious development of a terminal rhizome bud (rb). The rhizome bud forms in the axil of a scale leaf on the ventral surface of the rhizome. The plane of distichy of the scale leaves is vertical. 1D. Subsequent development of an erect shoot and rhizome buds as shown in preceding view at C. 1E. Later development of erect shoot (es) and rhizome (1). Note development of the two axes are about equal. 1F. General view of rhizomes arising from erect shoots. Note erect shoot (to right) gave rise to two rhizomes. Transitional leaves (t) (scale leaves) accompany early development of erect shoot. 1G. Erect shoot developing after seven nodes and internodes are formed by rhizome apex. Non-branching anchoring roots (nr) arise from nodes on the ventral surface of rhizome.

MORPHOLOGY AND ANATOMYOF JUNCUS ROEMERIANUS 5

The Roots is frequently surrounded on the outside by sclerenchyma or thick-walled parenchyma cells. Fibers are often found in There are two types of roots found in J. roemerianus. the phloem. A series of branching parenchyma cells radiate One type branches frequently, is relatively small in diameter from the sclerenchyma girders to the vascular bundles. and somewhat stiff. The other type generally does not These cells are stellate, a highly specialized polyhedral cell branch or does so infrequently. These roots are relatively type, and form transverse diaphragms or septa within the large, succulent, deeply penetrating and consistently arise leaf (Figure 3D). from the lower surface of the rhizome near the apex. The development of fibrous roots is somewhat delayed The Culm and restricted to the lateral surface (perpendicular to the plane of distichy) of the mature erect shoot (Figure 2-F). The flowering stem or culm is weakly developed and easily broken. Chlorenchyma occurs beneath the single layer VEGETATIVE ANATOMY epidermis. It is composed of irregular or rounded cells, two to three cells in thickness, and a few strands of sclerenchyma. The Leaf These sclerenchyma girders are much smaller than those found in the leaf. The culm center is composed of continuous The mature leaf blade is symmetrical in cross section parenchyma. A series of radiating diaphragms of stellate, (Figure 3A). There is a single-layered epidermis with the lobed cells or branching parenchyma are also found in the outer cell wall thickened and heavily cutinized. Stomata culm. The vascular bundles are fewer in number but larger are not recessed. The stomatal cavity or substomatal cham- in size in comparison to those of the leaf (Figure 4A and B). ber occurs in the palisade at a depth of one to several cells. However, the orientation of the phloem is the same as for Procumbent protective cells line the substomatal cavity the leaf, being external to the xylem. The stomata are not (Figure 3B and C). sunken. Protective cells are found lining the substomatal The chlorenchyma is composed of six to seven layers cavity. The isodametric cells of the chlorenchyma are loosely of elongated palisade cells with slightly swollen, pointed arranged, which allows for the presence of air cavities ends. The cells, more or less perpendicular to the leaf (Figure 4C and D). surface, appear to radiate from sclerenchyma girders or strands which occur next to the epidermis in triangular or The Rhizome irregular patches in cross section. These strands are often close together, separated by the width of the stoma only The abaxial side of the rhizome scale or cataphylls con- (Figure 3C-F). Other strands of sclerenchyma, more or tains an epidermis covered by a thick layer of cutin. The less rounded or oval in outline, are scattered in the central epidermis of the scale is one cell in thickness, but the cells ground tissue especially toward the solid parenchyma leaf are very large. A parenchyma layer from several to 20 cells center where each is surrounded by a parenchyma sheath. thickmay be found between the upper and lower epidermis. Both inner sclerenchyma and outer parenchyma bundle The rhizome has an epidermis of one cell in thickness which sheaths surround the vascular bundles. often contains lignin. Under the epidermis occurs a many- The vascular bundles generally occur in two or three layered hypodermis which generally varies in thickness along rings in the cylindrical leaves. Large vascular bundles with a single rhizome or between rhizomes of different habitats. two medium-sized metaxylem vessels on either flank occur The outer cell layers of the hypodermis are often lignified in the leaf center. The smaller bundles occur outermost and filled with an unidentified material. Lignification ac- and may be arranged in two or more rings, some free in companied by apparent suberization is pronounced in older the central ground tissue of the culm. However, most of rhizome portions. Under the hypodermis is a thick layer of the vascular bundles are concentrated near the periphery loosely arranged parenchyma, considered to be cortex. The of the leaf. The phloem of the vascular bundles is external innermost layer of the cortex is a single endodermoid layer. to the xylem and the arrangement of the vascular tissue In older portions of the rhizome, endodermoid cells have within the leaf resembles that of an atactostele. The phloem thickened inner walls. A pericycle, a layer of parenchyma

Figure 2. Vegetative morphology of Juncus roemeriunus. 2A. Erect shoot and associated rhizomes and rhizome buds. Rhizome (r) was formed simultaneously with erect shoot (es). Rhizome bud (rbl) arises dorsally on the rhizome slightly behind the erect shoot (es). Rhizome bud (rb2) arises from the erect shoot. Note that the buds at rbl and rb2 are forming erect shoots immediately after being formed. 2B. Erect shoot (esl) and associated erect shoots (es2) which arose from buds as described at A. 2C. Secondary rhizome developing erect shoot and rhizome bud which forms a further unit of the sympodium. 2D. Proliferation of erect shoots on colonizing plants growing without competition. Note that rhizome buds form erect shoots immediately. This example shows that elongated rhizomes between erect shoots are not formed in all instances. 2E. Distance (d) between erect shoots of mature plants varies between habitats. The segment of rhizome on the left has about 14 nodes and internodes separating erect shoots. This plant grew under conditions which apparently favored vegetative reproduction. The leaves were over 7 feet in length. The segment on the right has only a few nodes and compressed internodes between erect shoots. This plant grew under hypersaline conditions. 2F. Location of fibrous roots (fr) on erect shoot (es). These roots generally arise perpendicular to the plane of distichy on the lateral side of the erect shoot. Nonfibrous (nf) or anchoring roots arise from the ventral surface of the rhizome. :: ::

Figure 3. Anatomy of the leaf of Junm roemerimus. 3A. Transection of leaf. Note the thick layer of palisade cells (p) forming chlorenchyma and bundles (v). 38. vascular Leaf cross section showing sclerenchyma girders appressed against the epidermis and stoma Note the procumbent protective lining the substomatal chamber. of leaf ahowing general of (st) cells 3C Transection mangement palisade cells (p) between sclerenchyma girders and stomata (st). Vascular bundle (v) gives general view and shows phloem oriented external section(SF of series palisade (p). to the xylem. 3D. Longitudinal leaf showing a of stomata on the extreme left underlaid by cells A diaphragm (d) or septum of stcllate cells or branching parenchyma (polyhedrats). 3E and E. Cangitudinal sections showing the regu- lar arrangement of parisade cells Ip). sclerenchyma girder (6) and vascular bundle (v). Figure 4. Anatomy of the culm of Juncus roemeriunus. Transection of culm. Note arrangement of vascular bundles (v), diaphragms of stellate cells or branching parenchyma (b) and non-branching parenchyma (n). Sclerenchyma girders (s) are very small in comparison to those c)f the leaf. 4B. Close view of leaf in cross section showing sclerenchyma (s) and absence of palisade cells. Irregular or rounded CI?LIS form a thin layer of chlorenchyma (c). Note stellate cells (b) of diaphragm and extension to vascular bundles (v). 4C. Greater magnifi ca- tion of ’ leaf cross section showing stomata with guard cells (g) and thin layer of chlorenchyma (c). 4D. Longitudinal section of leaf shcIW- ing oricmtation of stomata and guard cells (g). Note loose arrangement of chlorenchyma. 8 ELEUTERIUS

tissue two to five cells in thickness, occurs immediately earlier supposition was incorrect and that sympodial branch- inside the endodermal layer. The center of the rhizome ing in most mature plants of J. roemerianus involved pre- contains numerous amphivasal vascular bundles surrounded cocious rhizome bud formation at the rhizome apex. This by parenchyma cells. Generally more vascular bundles are precocious bud formation is followed by vigorous rhizome found near the outer portion of the stele than in the center. growth, thus obscuring the clear-cut example of sympodial The smaller vascular bundles, often tortuously fusing or branching exhibited by immature and certain mature plants. dividing, occur immediately to the inside of the pericycle. The elongated rhizomes formed between the erect stems The central ground tissue is parenchyma (Figure SA-C). are peculiar and unique since in most species of Juncus where branching is distinctly sympodial the rhizome seg- The Roots ment between erect shoots is rather short, resulting in a clumped habit. This is an important vegetative characteristic Although there are two types of roots, based on general which accounts, in part, for the domination ofJ. roemerianus morphology, no major anatomical differences were noted over large areas of salt marsh. in comparing the two types. The roots have an epidermal Sympodial branching implies that the plant is composed layer one cell in thickness. The cortex is composed of of units which are determinate and that further growth radiating plates of cells separated by air spaces or lacunae. continues from axillary buds. In J. roemerianus, buds con- The outer part of this cortex is composed of a layer of sistently arise in the axils of scale leaves on the ventral hexagonal cells, three to nine cells in thickness. The inner surface of the rhizome and frequently in the axils of part of the cortex is composed of a layer of rounded, thin- transitional and foliage leaves on the erect shoot. Richards walled cells, which becomes thicker in older roots. This and Clapham (1941) state that the continuation bud con- layer is three to eight cells thick. The endodermis is one sistently arises in the axil of the second scale leaf of Juncus cell in thickness, with very thick inner and anticlinal walls inflexus. Anderson (1974) states that it occurs in the ninth and thin outer walls. A pericycle, a layer one to three cells scale leaf of J. roemerianus. However, I find the ventral in thickness, occurs immediately within the endodermis. scale leaf axil, in which the continuation bud occurs, to be Phloem persists in mature roots as inconspicuous patches highly variable. Sometimes it occurs immediately at the first pressed against the pericycle. The entire center of the root node (where one node separates the erect shoots), while in appears to be composed of xylem. However, small, thick- other material the bud may be found in the axil of the walled cells which may be sclerified parenchyma, compose eighteenth scale leaf (18 nodes separating the erect shoots). the central portion of the xylem and from I to 24 large Thus, the formation of erect shoots and rhizome buds vessels arranged in a ring occur in the outer portion of the apparently does not occur after any fixed number of nodes. stele where they are sometimes accompanied by a central The number of rhizome nodes between erect shoots varies metaxylem vessel in larger roots (Figure 5D and E). In- among plants from different localities and habitats of J. dividual metaxylem vessels may be surrounded on the out- roemerianus. side by one layer of flattened tracheids (Figure 5F-G). In some populations of J. roemerianus rhizome growth is suppressed but the leaves are very long. In other areas DISCUSSION the leaves may be relatively short, but rhizome growth is extensive. Various combinations of growth patterns exist in In an earlier account it was suggested that J. roemerianus different habitats. In some areas of relatively low salinity, possesses monopodial branching (Eleuterius 1975). This plants are often very robust with leaves over 7 feet in was based on early stages of development in mature rhizome length, whereas, in hypersaline areas the plants are dwarfed apices which showed where the meristem divided equally (less than 1 foot). Whether this variation in the size of or nearly so with the resulting dorsal meristem often slightly these plants is a response to environmental factors or due larger than the ventral. The lower meristem gives rise to the to genetic control remains unclear. However, data from dominant axis, the rhizome. In most cases the horizontal reciprocal transplant experiments suggest that ecotypic stem clearly exceeds the development of the erect stem. adaptation may exist within J. roemerianus. Reduced plant This prevailing condition suggested that two shoots had been size and increased vascularization of internal tissues is initiated simultaneously, the horizontal shoot or rhizome characteristic of plants which grow under saline conditions. being indeterminate. The branching is thus monopodial. However, the internal anatomy of various plant structures Subsequently, a more detailed investigation showed that the remains relatively constant. In dwarf plants the internal

Figure 5. Anatomy of the rhizome and roots of Juncus roemerianus. Transection of young rhizome showing arrangement and general appearance of tissues. The epidermis is a single layer of cells. The hypodermis (h), cortex (c) and endodermoid layer (e) are shown. The stele is composed of scattered amphivasal bundles. 5B. Transection of older rhizome showing well developed tissues. Note the lignified hypodermal cells (h) which may contain suberin. The cortex (c) and endodermoid layer (e) are well developed. The amphivasal bundles are well defined and are tightly appressed against the pericycle. 5C. An individual amphivasal bundle of rhizome stele. 5D. A cross section of a root. Note the large lacunae of the inner cortex, thick outer cortex and the relatively small vascular stele. 5E. Greater magnification of cross section showing detail of endodermis and vascular stele. Note the thick inner and anticlinal walls and thin outer walls of the cells of Another section of a root showing the endodermis with cell the endodermis. The metaxylem vessels are arranged in an outer ring. 5F. walls thicker than shown in E. This thickening appears to occur with age. 5G. Metaxylem vessel surrounded by flattened tracheids. MORPHOLOGYAND ANATOMYOF JUNCUS ROEMERIANUS 9 10 ELEUTERIUS tissues are correspondingly reduced with an obvious reduc- Internal anatomy has much use in taxonomic work. The tion in the amount of parenchyma which occurs in all presence of pith in the leaf was the characteristic used by plant organs. The amount of parenchyma is partially re- Buchenau (1 906) in defining Juncus subgenus Thalassi. stored in new organs of dwarf transplants when they are Raunkiaer (1895) placed J. maritimus in a separate category taken from hypersaline marshes and grown under low because the vascular bundles of the leaf and culm are found salinity. throughout the culm and leaf center. Adamson (1925) and The occurrence of stellate cells in the pith of the cos- Cutler (1969) showed that the leaf anatomy of J. maritimus mopolitan Juncus effusus has been described in detail by and J. acufus are essentially the same. Anatomically J. Maas Geesteranus (1941). In J. roemerianus, Juncus mariti- roemerianus resembles these two species of Juncus which mus Lam. and Juncus acutus L. these stellate cells form have solid piths with vascular bundles. The chief anatomical diaphragms and are regarded as a secondary development in differences which exist between J. maritimus, J. acutus and the more advanced species (Buchenau 1906). Diaphragms J. roemerianus are found in transverse sections of the leaf. have been found in other water plants (Kaul 1971, 1973, The detailed morphological and anatomical characteristics 1974) associated with air chambers which provide internal of J. roemerianus described in this paper may have taxo- ventilation. These diaphragms probably give strength with- nomic value in diagnostic work, especially where hybrids are out rigidity (Snow 1914; Williams and Barber 1961). concerned or suspected (Stace 1970; Eleuterius 1975). How- Several morphological and anatomical features are ap- ever, no differences in vegetative anatomy were noted within parently related to water regulation. These include the J. roemerianus when plants which produce perfect and pistil- presence of lignin and possible suberin in the outer layer of late flowers were compared (Eleuterius and McDaniel 1974). the hypodermis and cortex of the rhizome and roots, the well-developed endodermis of the roots and endodermoid ACKNOWLEDGEMENT layer of the rhizomes, the heavy cutinization of the rhizomes, scales and foliage leaves, and the fibrous leaves. I thank Dr. Robert B. Kaul, Botany Department, Uni- Clements (1907) stated that in grasses and sedges the versity of Nebraska, for his constructive comments regard- presence of large amounts of sclerenchyma in the leaves ing this work and manuscript. I thank Roberta Langenfeld renders water loss difficult. for typing and proofreading.

LITERATURE CITED

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